A Two-Year Surveillance of 2009 Pandemic Influenza A (H1N1) in Guangzhou, China: From Pandemic to Seasonal Influenza?

In this two-years surveillance of 2009 pandemic influenza A (H1N1) (pH1N1) in Guangzhou, China, we reported here that the scale and duration of pH1N1 outbreaks, severe disease and fatality rates of pH1N1 patients were significantly lower or shorter in the second epidemic year (May 2010-April 2011) than those in the first epidemic year (May 2009-April 2010) (P<0.05), but similar to those of seasonal influenza (P>0.05). Similar to seasonal influenza, pre-existing chronic pulmonary diseases was a risk factor associated with fatal cases of pH1N1 influenza. Different from seasonal influenza, which occurred in spring/summer seasons annually, pH1N1 influenza mainly occurred in autumn/winter seasons in the first epidemic year, but prolonged to winter/spring season in the second epidemic year. The information suggests a tendency that the epidemics of pH1N1 influenza may probably further shift to spring/summer seasons and become a predominant subtype of seasonal influenza in coming years in Guangzhou, China

analysis of potential risk factors associated with the death of pH1N1 influenza severe cases in the first epidemic year.^*

<p>*Four pH1N1 sever cases were reported in the second epidemic year with one death and the dead case also had pre-existing chronic pulmonary disease.</p>#<p>OR = odds ratio.</p

Seasonal distribution of influenza in Guangzhou from May 2008 to April 2011.

<p>(A) Confirmed cases of seasonal influenza (Seasonal, including H1N1, H3N2 and B) and pH1N1 influenza are shown in the indicated time. (B) Confirmed cases of seasonal influenza caused by subtypes H1N1, H3N2 and B are shown in the indicated time.</p

Severe disease and fatality rates of pH1N1 and seasonal influenza in Guangzhou.

<p>(A) Severe disease rate (% of patients progressed to severe disease) of pH1N1 influenza in the first (1st pH1N1) and second (2nd pH1N1) epidemic year and of seasonal influenza in the second epidemic year (2nd seasonal). (B) Fatality rate (deaths/10000) of pH1N1 influenza in the first (1st pH1N1) and second (2nd pH1N1) epidemic year and of seasonal influenza in the second year (2nd seasonal).</p

The frequency, scale and duration of outbreaks of influenza in the first and second epidemic year.

<p>*Average number of outbreaks/month.</p>&<p>Average attck rate/outbreak (%).</p>#<p>Average days/outbreak±SD.</p